Remotely controllable defect detector of electric resistance type

ABSTRACT

A remotely controllable defect detector, of the electric resistance type, includes a relatively elongated control rod, a guide frame supported at one end of the rod and a contact assembly including a body member supported by the frame. The body member carries a pair of spaced electric current probes for contact with spaced points of an electrically conductive object or body to be checked, to provide a flow of current through the checked body between the contact points, and the body member carries a pair of spaced voltage probes for contact with the checked body to detect the potential difference between points of the checked body. The contact assembly is preferably adjustably mounted on the guide frame, and the guide frame is adjustable relative to the control rod. Control means are provided at the end of the control rod remote from the end thereof carrying the guide frame, and are operable to adjust the contact assembly relative to the control rod. The control means may include, for example, a manually operable member connected to gearing operable to rotate the contact assembly about an axis perpendicular to the control rod. A hydraulic pressure actuator or an electromagnet may be used to press the guide frame firmly into engagement with the body to be checked.

Shigematsu et al.

[ Oct. 17, 1972 [54] REMOTELY CONTROLLABLE DEFECT DETECTOR OF ELECTRICRESISTANCE TYPE [72] Inventors: Hiroji Shigematsu; Tomio Yamaguchi, bothof Kobe; Hisanobu Fukue, Kakogawa; Hiroshi Ogawa; Takashi l-lashimoto;Kenichi Murakoshi; Susumu Suguri, all of Mito; Tsutomu Fujimura, Tokai,all of Japan [731 Assignees: Mitsubishi Jukogyo Kabushiki Kaisha; JapanAtomic Energy Research Institute, Tokyo, Japan [22] Filed: Feb. 24, 1970[21] Appl. No.: 13,292

[52] US. Cl ..324/65 R, 324/64, 324/725,

[51] Int. Cl. ..G0lr 27/08 [58] Field of Search ..324/61 P, 65 CP, 65CR, 64,

[56] References Cited UNITED STATES PATENTS 2,459,196 l/1949 Stewart..324/64 X 2,843,823 7/1958 Bayless ..324/72.5 X

2,476,943 7/1949 Brady ..324/64 2,440,044 4/1948 Greenslade ..324/64Primary Examiner-Herman Karl Saalbach Assistant Examiner--Paul L.Gensler Att0rneyMcGlew and Toren [5 7] ABSTRACT A remotely controllabledefect detector, of the electric resistance type, includes a relativelyelongated control rod, a guide frame supported at one end of the rod anda contact assembly including a body member supported by the frame. Thebody member carries a pair of spaced electric current probes for contactwith spaced points of an electrically conductive object or body to bechecked, to provide a flow of current through the checked body betweenthe contact points, and the body member carries a pair of spaced voltageprobes for contact with the checked body to detect the potentialdifference between points of the checked body. The contact assembly ispreferably adjustably mounted on the guide frame, and the guide frame isadjustable relative to the control rod. Control means are provided atthe end of the control rod remote from the end thereof carrying theguide frame, and are operable to adjust the contact assembly relative tothe control rod. The control means may include, for example, a manuallyoperable member connected to gearing operable to rotate the contactassembly about an axis perpendicular to the control rod. A hydraulicpressure actuator or an electromagnet may be used to press the guideframe firmly into engagement with the body to be checked.

13 Claims, 7 Drawing Figures REMOTELY CONTROLLABLE DEFECT DETECTOR OFELECTRIC RESISTANCE TYPE BACKGROUND OF THE INVENTION trically conductivebody for defects and the like, the

tips of the probes are brought into contact with the test body and theelectric current probes have a potential supplied thereto so that apredetermined direct current will flow through the conductive bodybetween the current probes. At the same time, the potential differencebetween the voltage probes is indicated by the voltmeter.

A calibration curve representing the correlation between the potentialdifference and the depth of a defect is prepared beforehand on the basisof data determined from test pieces of the same material and havingdefects at known depths. Then, by comparing the measured potentialdifference between the voltage probes to predetermine potentialdifferences on the calibration curve, the actual depth of any defect inthe body being tested can be read and detected.

In recent years, for example, in the periodic inspection of the pressurevessel of a nuclear reactor, increasing attention has been paid to thenecessity of checking the pressure vessel, particularly such portionsthereof as the nozzle for forced circulation of coolant and also thebottom of the reactor, which are subjected to relatively high stresses,to determine if there are any defects and what is the extent of anydefect, in order to ensure the safety of the pressure vessel. However,the radioactive radiation level within the vessel is so high that thevessel usually must be filled with water. In addition, the presence ofthe nozzle for forced circulation of the coolant, and the reactorbottom, which are located in excess of 10 meters below the top end ofthe pressure vessel body, as well as the core structure, prevent accessof operators to these parts for inspection. The only alternative is tomake such an inspection under remote control. Despite this, none of theknown instruments of the type described above is capable of beingremotely controlled.

SUMMARY OF THE INVENTION This invention relates to defect detectors ofthe electric resistance type and, more particularly, to a novel andimproved remotely controllable defect detector for detecting defects atsubstantially inaccessible locations in an electrically conductivemember or body to be checked.

The objective of the present invention is to provide such an instrumentwith which it is possible to detect any crack, flaw, or other defect ofan electrically conductive body to be inspected, to determine the depth,length, etc., of any such defect, all by remote control, and to select,by remote control, the region to be checked. For this purpose, aremotely controllable defeet detector embodying the invention comprises,in essence, a contact assembly including a pair of electric currentprobes, arranged to be held in contact with an electrically conductivebody being checked to provide a flow of current therethrough, and a pairof electric voltage probes in contact with the body to be checked, todetect the potential difference between points thereof. The detectorfurther includes a guide frame supporting the main body member of thecontact assembly, and a control rod that supports, at its lower end,this guide frame. The contact assembly and the guide frame form oneassembly, and the guide frame and the control rod form a secondassembly, and at least one of these two assemblies is adjustable ormovable relative to the other assembly.

An object of the invention is to provide a defect detector of thecharacter described which is capable of remote control, so that thecontact assembly, with the current and voltage probes, can beefficiently pressed against an electrically conductive body beingchecked, whereby to ensure positive contact between the probes and theelectrically conductive body.

Another object of the invention is to provide such a defect detector inwhich not only is the contact assembly pressed sufficiently against theelectrically conductive body being checked but also the contact positionof the contact assembly, with respect to the electrically conductivebody, can be shifted with ease.

A further object of the invention is to provide a remotely controllabledefect detector of the electrically resistance type which is simple inconstruction, economical to manufacture, and efficient in operation.

For an understanding of the principles of the invention, reference ismade to the following description of typical embodiments thereof asillustrated in the accompanying drawings:

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a somewhat schematic vertical sectional view illustrating oneembodiment of a defect detector in accordance with the invention as usedfor detecting defects in the nozzle part of a pressure vessel of anuclear reactor;

FIG. 2 is a vertical sectional view, to a substantially enlarged scale,of the lower portion of the defect detector as applied to the pressurevessel;

FIG. 3 is a vertical elevation view taken at substantially right anglesto the plane of FIG. 2;

FIG. 4 is a longitudinal sectional view of a contact assembly formingpart of the defect detector;

FIG. 5 is a partial vertical sectional view illustrating anotherembodiment of the invention; and

FIGS. 6 and 7 are views explanatory of another embodiment of contactassembly in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIG. 1, anelectrically conductive body 1 to be checked is illustrated as apressure vessel for a nuclear reactor, with the core structure, insidepressure vessel 1, being generally indicated at 2. Vessel 1 includes anozzle 3 adjacent its lower end for forced circulation of cooling water.The contact assembly of a remotely controllable defect detectorembodying the invention is illustrated at A, and is shown in detail inFIG. 4.

Referring to FIG. 4, contact assembly A includes a cylindrical bodymember 4 consisting of an outer cylindrical body section 4a and an innercylindrical body section 4b, the adjacent ends of the cylindrical bodysections being disengageably connected to each other. The outer portionof body section 4a is provided with an inner cylinder 5 which isconcentric and integral therewith. A piston 6 is slidable in bodysection 4a, between the inner surface of this body section and the outersurface of cylinder 5, and O-rings 7 are mounted on piston 6 to form aseal between this piston, the inner surface of body section 4a and theouter surface of inner cylinder 5. By virtue of the sealing 0 rings 7,fluid chamber 8 is defined between the head 6a of piston 6, the innerside wall and the outer end wall of body section 4a, and the outersurface of inner cylinder 5. Fluid chamber 8 and piston 6 constitute ahydraulic actuator, and working fluid is supplied to chamber 8 throughan inlet port 9 arranged to be connected to a suitable source ofhydraulic fluid under pressure.

A cylindrical part or support 10, of electrical insulating or dielectricmaterial, carries a pair of electric current probles 11, 11' and a pairof voltage probes 12, 12. Support 10 has a reduced diameter portion 10aat its lower end and has a bottom wall 10b. At the upper end of support10,.there is a disk 10d of electrical insulating or dielectric material,which is disengageably fitted in a groove at the top periphery 100 ofsupport 10. Four bores We are formed equidistantly through the bottomwall 10b of support 10, and are coaxially aligned with four holes 10fformed in disk 10d. The bores 10c and the holes 10f receive the probes11, 11' and l2, 12 in a manner such that voltage probes 12, 12 arelocated intermediate current probes 11, 11, with the upper ends of allof the probles extending beyond disk 10d.

Springs 14 embrace the probes between flanges 13 thereon and disk d,and, under the bias of these springs, the probes are held in place and,if there is some irregularity in the surface to be checked, theindividual probes can be relatively displaced in conformity to theparticular configuration of the surface to be checked, so as to maintaingood contact between the probes and such surface. A larger coil spring15 is disposed between the step, formed by the reduced diameter portion10a at the lower part of support 10, and a radially inwardly directedflange 16 at the inner end of inner body section 4b. Spring 15 serves tobias support 10 against the inner end of piston 6, so that probes 11,11' and 12, 12' are maintained retracted within body member 4 when thehydraulic actuator is not operative.

The surfaces of probes 11, 11' and 12, 12' may be coated with a suitableelectrically insulating paint to ensure good electrical insulation. Thisis particularly desirable in the inspection, for example, of a pressurevessel of a nuclear reactor and which must be filled with water to avoidradioactive hazards.

Referring again to FIGS. 1 to 4, a control rod for a guide frame 17 isindicated at B, and comprises a very substantially elongated outer tube18 at the bottom end of which a sleeve 20, for a hydraulic jack 19, issupported by support plates 18a secured to tubing 18, and

also by collars 20a extending around sleeve 19, the support being suchthat sleeve 20 can rotate about an axis perpendicular to the axis oftubing 18 but that sleeve 20 cannot move in an axial direction.

A control mechanism is provided at the upper end of tubing 18 of controlrod B, and inside the tubing, and this control mechanism comprises ahandwheel 21, a connecting rod or shaft 22, bevel gears 23, 24, a worm25 and a worm gear 26 so that, by maneuvering handwheel 21, hydraulicjack 19 is remotely controlled and sleeve 20 of jack 19 may be rotated.

A guide sleeve or frame 17 supports contact assembly A in position, andthis guide sleeve is coupled, at its center, by a pin 27 to the outerend of sleeve 20 of hydraulic jack 19. Accordingly, guide frame 17rotates together with sleeve 20, thereby effecting rotation orrevolution of probes 11, 11' and l2, 12' of contact assembly A supportedby frame 17, to provide for shifting the position of the contactassembly with respect to the surface of the conductive body beingchecked.

The portion of guide frame 17 opposite the surface to be checked has acontour such as to mate substantially with the contours of the opposingsurface, so that the guide frame can closely approach the oppositesurface. In the particular embodiments shown in the drawings, thecontour of the guide frame is made to approach mating relation with themouth shape of nozzle 3. On both sides, of the opposite ends of thatsurface of guide frame 17 opposite the surface being checked, there areformed protrusions 17b, 17b which maintain a limited spacing betweenguide frame 17 an the surface being checked.

As more particularly illustrated in FIG. 2, contact assembly A ismounted on guide frame 17 in a manner such as to be shiftable andadjustable in position, by means of a support arm 17a in engagement witha guide slot 17c formed in frame 17. Shifting and adjustment of theposition, and fixing, of support arm 17a, are remotely controlled by asuitable drive mechanism and a suitable control mechanism which areremotely located and which have not been illustrated. However, while itis desirable, from the standpoint of operating efficiency, to controlremotely the shifting and adjustment of the position, and the fixing inposition, of support arm 17a, a remote control is not always essential.

For example, contact assembly A may be so constructed that it can beelevated to the control stand and then support arm 17a can be manuallyshifted or adjusted in position or fixed, as by bolting, on guide frameHydraulic jack 19 is remotely actuated by fluid pressure from a remotelylocated source of hydraulic fluid under pressure, and head 19a of jack19 is brought into contact with core structure 2 of the electricallyconductive body being checked, such as the pressure vessel 1 for anuclear reactor. This produces a reaction force which, in turn, pressesprotrusions 17b, 17b of guide frame 17 against the surface of theelectrically conductive body, for example, against the surface ofpressure vessel 1 for a nuclear reactor, which body is being checked sothat guide frame 17 is suitably positioned.

In the embodiment illustrated, positioning of guide frame 17 isaccomplished by hydraulic jack 19 where there is a stationary object,such as core structure 2, inside the electrically conductive body beinginspected.

In the absence of any such stationary internal structure which can serveas a support, hydraulic jack 19 may be replaced by an electromagnet M,as shown in FIG. 5, and which is secured to a suitable point along outertubing 18 and facing guide frame 17. Electromagnet M is excited from apower source which is remotely located, so that the resultant magneticattraction positions guide frame 17 in contact with the surface to bechecked. A book 28 is secured to the upper end of outer tubing 18 forcontrol rod B.

A defect detector embodying the invention, and constructed as so fardescribed, is operated in a manner which will now be set forth. Toinspect a given part of an electrically conductive body such as nozzle 3of pressure vessel 1 in the embodiment illustrated, control rod B isinitially suspended by hook28 from a suitable crane, and is lowered to asuitable point inside pressure vessel 1. Following this, control rod Bis suitably adjusted up or down, using the crane, until the central partof guide frame 17 for contact assembly A is in a position in alignmentwith the center of nozzle 3. Insertion and vertical adjustment ofcontrol rod B need not always be effected by using a crane, but may beaccomplished by other suitable hoisting or conveying means of a knowntype. Hydraulic fluid under pressure, from a remotely located source, isthen supplied to hydraulic jack 19 to actuate this jack to bring jackhead 19a into contact with core structure 2 and, at the same time, allowthe reaction force to urge protrusions 17b, 17b of guide frame 17 intocontact with the surface of nozzle 3 to be checked.

Following this, hydraulic fluid under pressure is supplied to thehydraulic actuator, or fluid chamber 8, of contact assembly A and, bymeans of piston 6, support is caused to move outward in cylindrical body4 until the tips of probes ll, 11' and 12, 12 are moved into contactwith the surface to be checked. Since probes 11, 11 and 12, 12 areslidable in bores 6a of support 10, under the bias of springs 14, thetips of the probes are kept most satisfactorily in contact with thesurface of the conductive body regardless of any surface irregularitythereof. A predetermined direct current from a remote source ofpotential is then supplied to current probes ll, 11, and the potentialdifference between voltage probes 12, 12 is detected by a remotelylocated voltmeter connected to these voltage probes. By this procedure,it can be determined or detected whether there is any crack, flaw, orother defect in the part being checked, and there can also be determinedhow deep or how long any flaw is, such detection being carried outexactly at the remote point.

Other parts of nozzle 3 are scanned in the following manner. The fluidpressures in the hydraulic actuator of the contact assembly and inhydraulic jack 19 are released, and probes 11, 11 and 12, 12 andprotrusions 17b, 17b of guide frame 17, are retracted out of contactwith the surface of the conductive body being checked. l-Iandwheel 21then is maneuvered to rotate guide frame 17 through the controlmechanism and the sleeve of hydraulic jack 19, so as to position theguide frame as desired. If necessary, support arm 17a of guide frame 17may be adjusted along slot 17c remotely by suitable means, or the probes11, 11 and 12, 12 may be manually positioned. Thereafter, the alreadydescribed procedure is repeated.

While, in the embodiment of the invention so far T described, contactassembly A is provided with a single pair of current probes 1 1, 11' anda single pair of voltage probes 12, 12 the contact assembly may bedesigned and constructed, as will now be described, for greaterfacilitating of the defect detection and determination. Such anembodiment is illustrated in FIGS. 6 and 7.

Referring to FIGS. 6 and 7, a contact assembly A is provided with aplurality of probes b b b etc. A selecting switch C is connected byleads d d d etc., to the respective probes b b b etc., and is alsoconnected to a source or potential e, by leads 3, and to a voltmeter f,by leads h. Although, in FIG. 6, the probes b b b etc. are shown as ofthe same length, the probe lengths may be varied to best meet thecontour of the part being checked, as shown in FIG. 7, in order toassure good contact and thus easy and positive detection.

In detecting and determining a crack, flaw, or other detect in anelectrically conductive body, using the defect detector shown in FIGS. 6and 7, contact assembly A is so positioned that probes b b b etc., areopposed to the surface to be checked and are maintained in contact withthe latter. Then selecting switch C is set so as to connect a selectedpair or probes to potential source e, and also to connect anotherselected pair of probes, disposed between the first selected pair, tovoltmeter f. Thereby, a predetermined direct current from source e flowsthrough the pair of probes connected to this source. The potentialdifference between the probes connected to voltmeter f is then detectedby the voltmeter and, using this procedure, it can be determined whetherthere is any crack and, if there is a crack or flaw, how deep and howlong the crack or flaw is, such detection being effected in the usualmanner. In order to check any other part adjacent the part alreadychecked, all that is necessary is to set the selecting switch C so thatfour probes come to face the unchecked part and function in the samemanner as already described. Thereby, another part of the body to bechecked can be checked out.

By repeating the above procedure while contact assembly A is maintainedin its original position, it is possible to determine if there is anycrack, flaw, or other defect on the entire surface of the electricallyconductive body that is scanned by the probes b b b b, and, if there areany cracks, flaws or defects to determine the depth, length, etc.thereof. Thus, once assembly A is properly positioned, defect detectionwith respect to an electrically conductive body can be effectedcontinuously over a fairly extensive area proportional to the number ofprobes b b b etc. available in contact assembly A. Thereby, the timerequired for detection of defects can be greatly reduced. Even where anypart being checked is surrounded by various members or elements whichotherwise hamper the detection operation, the instrument embodying theinvention enables detection and determination of defects to be performedin a simplified manner and with an extremely high degree of efficiency.

Summarizing, the defect detector embodying the invention comprises acontact assembly having a pair of electric current probes adapted to beheld in contact with an electrically conductive body being checked inorder to provide a flow of current through the body, and a pair ofelectric voltage probes in contact with the conductive body to detect apotential difference in the latter. A guide frame supports the main bodymember of the contact assembly in position, and this guide frame issupported at the lower or inner end of a control rod. At least eitherthe combination of the contact assembly and the guide frame, or thecombination of the guide frame and the control rod, are movable withrespect to each other. With the described construction, it is possible,under remote control, to detect if an electrically conductive body beingchecked has any crack, flaw or other defect and, if there is any suchcrack, flaw or defect, how deep and how long the same is. Additionally,the area to be checked can be shifted or adjusted by remote control. Theinstrument thus permits pinpoint detection and determination of anycrack, flaw, or other defect of an electrically conductive body, such asa pressure vessel for a nuclear reactor, which requires remotelycontrolled inspection. By suitably designing the contour of the guideframe, the instrument is easily adapted to scan structures havingcomplicated configurations. These advantageous features add to theindustrial utility of the defect detector of the present invention.

While specific embodiments of the invention have been shown anddescribed in order to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:

1. A remotely controllable defect detector, of the electric resistancetype, comprising, in combination, a relatively elongated control rod; aguide frame supported at one end of said control rod; a contact assemblyincluding a body member supported by said guide frame; at least a pairof spaced electric current probes carried by said body member forcontact with spaced points of an electrically conductive body to bechecked, and connectable to a source of electric'potential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said contact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; both said guide frame andsaid contact assembly being adjustable relative to said control rod.

2. A remotely controllable defect detector, as claimed in claim 1,including means operable to press said contact assembly against theelectrically conductive body to be checked.

3. A remotely controllable defect detector, as claimed in claim 2, inwhich said last named means comprises hydraulic actuator means.

4. A remotely controllable defect detector, as claimed in claim 3, inwhich said contact assembly includes a probe support mounted in saidbody member for displacement in the latter; said hydraulic actuatormeans including a hydraulic actuator in said body member including achamber formed in said body member, and operable on said support topress said probes into contact with the electrically conductive body tobe checked.

5. A remotely controllable defect detector, as claimed in claim 3,wherein said hydraulic actuator means includes a hydraulic actuator,having a fluid chamber, and operable to press said control rod towardthe electrically conductive body to be checked.

6. A remotely controllable defect detector, as claimed in claim 2, inwhich said last named means is operable to press the control rod towardthe electrically conductive body to be checked.

7. A remotely controllable defect detector, as claimed in claim 6, inwhich said last named means comprises a hydraulic cylinder and a pistonoperable in said cylinder.

8. A remotely controllable defect detector, as claimed in claim 6,wherein said last named means comprises an electromagnet secured to saidcontrol rod and operable, when energized, to exert a magnetomotive forcepressing said control rod toward the electrically conductive body to bechecked.

9. A remotely controllable defect detector, of the electric resistancetype, comprising, in combination, a relatively elongated control rod; aguide frame supported at one end of said control rod; a contact assemblyincluding a body member supported by said guide frame; at least a pairof spaced electric current probes carried by said body member forcontact with spaced points of an electrically conductive body to bechecked, and connectable to a source of electric p0tential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said contact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; said contact assemblybeing adjustable relative to said guide frame and relative to saidcontrol rod.

10. A remotely controllable defect detector, of the electric resistancetype, comprising, in combination, a relatively elongated control rod; aguide frame supported at one end of said control rod; a contact assemblyincluding a body member supported by said guide frame; at least a pairof spaced electric current probes carried by said body member forcontact with spaced points of an electrically conductive body to bechecked, and connectable to a source of electric potential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said con tact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; said guide frame beingformed with a guide slot; and means mounting said contact assembly foradjustment along said guide slot.

11. A remotely controllable defect detector, of the electric resistancetype, comprising, in combination, a relatively elongated control rod; aguide frame supported at one end of said control rod; a contact assemblyincluding a body member supported by said guide frame; at least a pairof spaced electric current probes carried by said body member forcontact with spacedpoints of an electrically conductive body to bechecked, and connectable to a source of electric potential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said contact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; and means mounting saidguide frame rotatably on said control rod.

12. A remotely controllable defect detector, of the electric resistancetype, comprising, in combination, a relatively elongated control rod; aguide frame sup ported at one end of said control rod; a contactassembly including a body member supported by said guide frame; at leasta pair of spaced electric current probes carried by said body member forcontact with spaced points of an electrically conductive body to bechecked, and connectable to a source of electric potential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said contact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; said contact assemblymounting a plurality of probes in ex-. cess of four; and selectingswitch means selectively operable to connect a first pair of said probesto a source of electric potential and to connect a second pair of saidprobes, intermediate the probes of said first pair, to a voltmeter.

13. A remotely controllable defect detector, as claimed in claim 12, inwhich said probes have different lengths in accordance with the contoursof the surface of the electrically conductive body to be checked.

1. A remotely controllable defect detector, of the electric resistancetype, comprising, in combination, a relatively elongated control rod; aguide frame supported at one end of said control rod; a contact assemblyincluding a body member supported by said guide frame; at least a pairof spaced electric current probes carried by said body member forcontact with spaced points of an electrically conductive body to bechecked, and connectable to a source of electric potential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said contact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; both said guide frame andsaid contact assembly being adjustable relative to said control rod. 2.A remotely controllable defect detector, as claimed in claim 1,including means operable to press said contact assembly against theelectrically conductive body to be checked.
 3. A remotely controllabledefect detector, as claimed in claim 2, in which said last named meanscomprises hydraulic actuator means.
 4. A remotely controllable defectdetector, as claimed in claim 3, in which said contact assembly includesa probe support mounted in said body member for displacement in thelatter; said hydraulic actuator means including a hydraulic actuator insaid body member including a chamber formed in said body member, andoperable on said support to press said probes into contact with theelectrically conductive body to be checked.
 5. A remotely controllabledefect detector, as claimed in claim 3, wherein said hydraulic actuatormeans includes a hydraulic actuator, having a fluid chamber, andoperable to press said control rod toward the electrically conductivebody to be checked.
 6. A remotely controllable defect detector, asclaimed in claim 2, in which said last named means is operable to pressthe control rod toward the electrically conductive body to be checked.7. A remotely controllable defect detector, as claimed in claim 6, inwhich said last named means comprises a hydraulic cylinder and a pistonoperable in said cylinder.
 8. A remotely controllable defect detector,as claimed in claim 6, wherein said last named means comprises anelectromagnet secured to said control rod and operable, when energized,to exert a magnetomotive force pressing said control rod toward theelectrically conductive body to be checked.
 9. A remotely controllabledefect detector, of the electric resistance type, comprising, incombination, a relatively elongated control rod; a guide frame supportedat one end of said control rod; a contact assembly including a bodymember supported by said guide frame; at least a pair of spaced electriccurrent probes carried by said body member for contact with spacedpoints of an electrically conductive body to be checked, and connectableto a source of electric potential for flow of current through the bodybetween the points; at least a pair of spaced voltage probes carried bysaid body member for contact with the checked body for detection of thepotential difference between points of the checked body; means mountingsaid contact assembly for aDjustment relative to said control rod; andcontrol means, at the end of said control rod remote from said one endthereof, operable to adjust said contact assembly relative to saidcontrol rod; said contact assembly being adjustable relative to saidguide frame and relative to said control rod.
 10. A remotelycontrollable defect detector, of the electric resistance type,comprising, in combination, a relatively elongated control rod; a guideframe supported at one end of said control rod; a contact assemblyincluding a body member supported by said guide frame; at least a pairof spaced electric current probes carried by said body member forcontact with spaced points of an electrically conductive body to bechecked, and connectable to a source of electric potential for flow ofcurrent through the body between the points; at least a pair of spacedvoltage probes carried by said body member for contact with the checkedbody for detection of the potential difference between points of thechecked body; means mounting said contact assembly for adjustmentrelative to said control rod; and control means, at the end of saidcontrol rod remote from said one end thereof, operable to adjust saidcontact assembly relative to said control rod; said guide frame beingformed with a guide slot; and means mounting said contact assembly foradjustment along said guide slot.
 11. A remotely controllable defectdetector, of the electric resistance type, comprising, in combination, arelatively elongated control rod; a guide frame supported at one end ofsaid control rod; a contact assembly including a body member supportedby said guide frame; at least a pair of spaced electric current probescarried by said body member for contact with spaced points of anelectrically conductive body to be checked, and connectable to a sourceof electric potential for flow of current through the body between thepoints; at least a pair of spaced voltage probes carried by said bodymember for contact with the checked body for detection of the potentialdifference between points of the checked body; means mounting saidcontact assembly for adjustment relative to said control rod; andcontrol means, at the end of said control rod remote from said one endthereof, operable to adjust said contact assembly relative to saidcontrol rod; and means mounting said guide frame rotatably on saidcontrol rod.
 12. A remotely controllable defect detector, of theelectric resistance type, comprising, in combination, a relativelyelongated control rod; a guide frame supported at one end of saidcontrol rod; a contact assembly including a body member supported bysaid guide frame; at least a pair of spaced electric current probescarried by said body member for contact with spaced points of anelectrically conductive body to be checked, and connectable to a sourceof electric potential for flow of current through the body between thepoints; at least a pair of spaced voltage probes carried by said bodymember for contact with the checked body for detection of the potentialdifference between points of the checked body; means mounting saidcontact assembly for adjustment relative to said control rod; andcontrol means, at the end of said control rod remote from said one endthereof, operable to adjust said contact assembly relative to saidcontrol rod; said contact assembly mounting a plurality of probes inexcess of four; and selecting switch means selectively operable toconnect a first pair of said probes to a source of electric potentialand to connect a second pair of said probes, intermediate the probes ofsaid first pair, to a voltmeter.
 13. A remotely controllable defectdetector, as claimed in claim 12, in which said probes have differentlengths in accordance with the contours of the surface of theelectrically conductive body to be checked.